We use oxygen-15 water positron emission tomography (PET) to measure regional cerebral blood flow (rCBF) as a marker of local neuronal activity in patients with schizophrenia and other neuropsychiatric disorders during performance of working memory and abstract reasoning tasks as well as during performance of matched sensorimotor control tests. Using a 2-back version of an N-back continuous working memory task, we have re-confirmed previous findings (derived with a more complex task, the Wisconsin Card Sorting Test) of hypofunction of prefrontal cortex. As in those previous studies, the aberrant activation pattern appears to occur even in some patients who perform relatively well on the task. Using single-subject activation mapping, a relatively new technology made feasible by 3-D PET scanning, we are investigating the relationship of activation signals for individual medication-free patients to the group activation signal. We are testing the hypotheses that 1) hypofunction of the prefrontal cortex will be observed in individual patients as well as for the group, and 2) in some patients, aberrant signals in regions not activated by the control subjects will appear, and these will vary from patient to patient. We have shown that the spatial extents of the individual subjects' activations are less that the control subjects and that the locales of the activations are more variable that the controls, at least when data are analyzed in stereotaxically standardized neuroanatomical space. We are currently performing further analyses of these data to determine whether the stereotaxic standardization plays a role in these results. In this study we are also examining changes in cognitively-evoked activation across several time points during a medication-free period in individual patients and determining the relationship between the physiological changes and concomitant changes in symptomatology and cognitive performance. Control subjects show considerable consistency of areas activated by the N-back task, both across subjects and across time points within subjects. In contrast, patients show a general picture of hypoactivation, but also show more heterogeneity across subjects and across time points. Further analyses of the timecourse of these changes are ongoing. We have also shown that even when patients with schizophrenia are compared with young normal subjects with equally poor performance, they still have diminished prefrontal cortical response while performing tasks with a strong working memory component. Moreover, patients with schizophrenia still show reduced activation of the prefrontal cortex when compared with healthy, elderly subjects matched with them on a one-to-one basis for WCST performance. In this schizophrenic group, reduced activation in the dorsolateral prefrontal cortex (DLPFC) correlated with more perseveration during the WCST. In contrast, in the elderly subjects DLPFC activation is preserved and does not correlate with performance, suggesting that other mechanisms may account for their cognitive impairment on this task. We have initiated a number of cross-modal neuroimaging studies in schizophrenia. We have found that prefrontal N-acetyl-aspartate magnetic resonance spectroscopy signal predicts impaired WCS rCBF activation not only in the prefrontal cortices of our patients, but also in other nodes in the working memory system. This relationship is not found in patients, but not in control subjects, and not with NAA in other brain regions for either group. In a study comparing IBZM binding (a SPECT measure that may reflect basal synaptic dopamine) with rCBF during working memory we found that dopamine receptor occupancy predicts hippocampal activation during working memory in schizophrenia.